The development of high energy battery systems requires, among other things, the compatibility of an electrolyte possessing desirable electrochemical properties with highly reactive anode materials, such as lithium or the like. The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. It has, therefore, been necessary, in order to realize the high energy density obtainable through use of these highly reactive anodes, to turn to the investigation of nonaqueous electrolyte systems.
The term "nonaqueous electrolyte" as used herein refers to an electrolyte which is composed of a solute, such as, for example, a metal salt or a complex salt of Group I-A, Group II-A, or Group III-A elements of the Periodic Table, dissolved in an appropriate nonaqueous solvent. The term "Periodic Table" as used herein refers to the Periodic Table of Elements as set forth on the inside back cover of the Handbook of Chemistry and Physics, 48th Edition, The Chemical Rubber Co., Cleveland, Ohio, 1967-1968.
A multitude of solutes is known and many have been suggested for use but the selection of a suitable solvent has been particularly troublesome. The ideal battery electrolyte would comprise a solvent-solute pair which has a long liquid range, high ionic conductivity and stability. A long liquid range, i.e., high boiling point and low freezing point, is essential if the battery is to operate at other than normal ambient temperatures. High ionic conductivity is necessary if the battery is to have high rate capability. Stability is necessary with the electrode materials, the materials of cell construction, and the products of the cell reaction to provide long shelf life when used in a primary or secondary battery system.
It has recently been disclosed in the literature that certain materials are capable of acting both as an electrolyte carrier, i.e., as solvent for the electrolyte salt, and as the active cathode for a non-aqueous electrochemical cell. U.S. Pat. Nos. 3,475,226, 3,567,515 and 3,578,500 each disclose that liquid sulfur dioxide or solutions of sulfur dioxide and a co-solvent will perform this dual function in nonaqueous electrochemical cells. While these solutions perform their dual function, they are not without several disadvantages in use. Sulfur dioxide is always present and, being a gas at ordinary temperatures, it must be contained in the cell as a liquid under pressure or dissolved in a liquid solvent. Handling and packaging problems are created if the sulfur dioxide is used alone, and an additional component and assembly step is necessary if sulfur dioxide is to be dissolved in a liquid solvent. As stated above, a long liquid range encompassing normal ambient temperatures is a desirable characteristic in an electrolyte solvent. Obviously, sulfur dioxide is deficient in this respect at atmospheric pressure.
U.S. application Ser. No. 439,521 by G. E. Blomgren et al, filed Feb. 4, 1974, which is a continuation-in-part of application Ser. No. 212,582 filed on Dec. 27, 1971 now abandoned, discloses a nonaqueous electrochemical cell comprising an anode, a cathode collector and a cathode-electrolyte, said cathode-electrolyte comprising a solution of an ionically conductive solute dissolved in an active cathode depolarizer wherein said active cathode depolarizer consists of a liquid oxyhalide of an element of Group V or Group VI of the Periodic Table. Although oxyhalides can be used effectively as a component part of a cathode-electrolyte in conjunction with an active metal anode, such as a lithium anode, to produce a good high energy density cell, it has been observed that if the cell is stored for a prolonged period of about 3 days or longer, passivation of the anode appears to occur which results in undesirable voltage delays at the beginning of discharge along with high cell impedance.
One of the primary objects of this invention is to substantially prevent the passivation of the active metal anode in oxyhalide cathode-electrolyte cells.
Another object of this invention is to provide an oxyhalide cathode-electrolyte cell wherein the surface of the active metal anode of the cell that is in contact with the liquid oxyhalide cathode-electrolyte is coated with a thin adherent vinyl polymer film so as to prevent the passivation of the active metal anode during cell storage and usage.
Another object of this invention is to provide a lithium-oxyhalide cell system wherein the surface of the lithium anode that is in contact with the liquid oxyhalide cathode-electrolyte of the cell is coated with a thin adherent vinyl polymer film so as to prevent the passivation of the lithium anode during cell storage and usage.